(1) Field of the Invention
This invention relates to a method and an apparatus for producing a silicon carbide single crystal. More particularly, it relates to a method and an apparatus for producing a silicon carbide single crystal wherein a silicon raw material is allowed to continuously react with a carbon raw material to generate gas, from which a silicon carbide single crystal grows.
The silicon carbide single crystal thus-produced has a large size and a high quality.
(2) Description of the Related Art
A silicon carbide single crystal expected for use as a semiconductor material is usually produced by a sublimation method using silicon carbide powder as a raw material. In the sublimation method, the raw material silicon carbide powder and a seed crystal substrate are disposed so as to face each other in a graphite reaction crucible, and the silicon carbide raw material is heated to a temperature in the range of 1,800xc2x0 C. to 2,400xc2x0 C. in an inert gas atmosphere. Sublimate gas of the silicon carbide generated by heating reaches the seed crystal substrate maintained at a temperature suitable for crystal growth, on which a silicon carbide single crystal grows. Sublimate gas components vary during the process of single crystal growth due to the sublimation and decomposition process of the silicon carbide powder as a raw material, and further mutual contact in the vapor phase and reaction of the sublimate gas components with graphite constituting the inner wall of the reaction crucible. As a method for suppressing and correcting such variation, a method for disposing a silicon raw material and a carbon raw material separately, and reacting gas components generated from the silicon raw material with the carbon raw material was proposed in, for example, Japanese Unexamined Patent Publication No. H6-316499.
Another proposed method is one for using silicon as a raw material, heating to evaporate silicon in the reaction crucible, reacting the generated silicon gas with carbon gas generated by evaporating the inner wall carbon of the reaction crucible, moving these to a silicon carbide deposition chamber, where a silicon carbide single crystal is deposited on the inner wall thereof (for example, Japanese Examined Patent Publication No. S51-8400). In the sublimation method, Si, Si2C, SiC2 and SiC are generated as sublimate and decomposed gases from the silicon carbide raw material, and composition of each of these sublimate gases varies in the sublimation process due to various factors. When the silicon carbide raw material is heated, the silicon component having a high vapor pressure is easily changed into gas, and the carbon remain conversely as a residual component. Consequently, with the passage of time, the silicon component in the raw material is reduced, and the gas composition in the sublimate gas is changed. This is one of the factors causing variation in the composition of the sublimate gas. A sublimation temperature of the raw material, a raw material composition, and a temperature distribution in the reaction crucible are also considered as other factors of variation. In the crystallization process of chemical species constituting the foregoing sublimate gas compositions to silicon carbides, reaction schemes are naturally different. Thus, the variation in the sublimate gas compositions in the single crystal growth process is considered to cause a reduction in a crystallinity by inclusion of a crystal defect in a crystal, and polymorphism intrusion. Therefore, to obtain a high-quality silicon carbide single crystal, the method of controlling such factors of variation is important.
In the present situation, however, since it is difficult to effect crystal growth by suppressing these factors to certain extents, the quality and stability of the silicon carbide single crystal obtained by the sublimation method, i.e., the method using the silicon carbide as a raw material are not satisfactory.
Also, to obtain a highly pure and high-quality single crystal for a semiconductor, it is necessary to use highly pure silicon carbide powder as a raw material, but the difficulty and high costs of obtaining such highly pure one are problems.
In a method using the silicon carbide powder as a raw material, a limitation is also placed on the weight of silicon carbide fed into the reaction crucible depending on a size of the reaction crucible. If the raw material is exhausted, batch processing of temporarily suspending the growth, lowering the temperature of the reaction crucible and then adding a new silicon carbide raw material must be carried out. Even if the silicon carbide raw material is fed without lowering the temperature of the reaction crucible, generally, a silicon component in the silicon carbide raw material is easily sublimated, decomposed or evaporated, and, with a progress of the sublimation, a carbon component is left. Consequently, continuous feeding of silicon carbide raw materials in the reaction crucible becomes impossible because of a capacity limitation of the reaction crucible.
In Japanese Unexamined Patent Publication No. H6-316499, silicon carbide is formed by allowing silicon to react with carbon, and then this silicon carbide is sublimated to form a silicon carbide single crystal. But an intrinsic drawback of the sublimation method, i.e., a change of a gas composition with the sublimation, is inevitable. In addition, a production method comprises two steps, and production time becomes relatively long. On the other hand, to continuously grow a crystal, the step of growing a crystal by elevating a temperature of the reaction crucible after the step of producing silicon carbide from silicon and carbon is repeated in batchwise fashion as in the case of the sublimation method. But because of an increasing or decreasing the temperature of the reaction crucible corresponding to each step, the crystal growth is not stable, and there is a possibility of causing distortion.
On the other hand, in the method of the above mentioned Japanese Examined Patent Publication No.S51-8400, silicon is used as a raw material, and silicon carbide is produced from silicon vapor produced therefrom and carbon vapor generated from the inner wall of the reaction crucible. But carbon has a low vapor pressure as compared with that of the silicon, and thus there is a drawback of a slow growth rate of a silicon carbide single crystal. In addition, since the carbon vapor generated from the inner wall of the reaction crucible is utilized for growth, if crystal growth is continued for a long time, the inner wall of the reaction crucible is reduced in thickness and, consequently, the crystal growth cannot be performed in continuous fashion.
An object of the invention is to provide a method for continuously growing a high-quality silicon carbide single crystal on a seed crystal substrate with good stability by continuously performing a reaction between silicon and carbon.
Another object of the invention is to provide an apparatus used for carrying out the above-mentioned method of the invention.
The present inventors made researches into the method for continuously growing a silicon carbide single crystal by utilizing gas generated by reacting silicon with carbon, and found that a high-quality silicon carbide single crystal can be obtained with little intrusion of elements or compounds other than the silicon carbide by continuous and quantitative feeding of silicon from the outside into a reaction crucible where silicon is placed into contact with carbon.
Thus, in one aspect of the present invention, there is provided a method for producing a silicon carbide single crystal comprising allowing a silicon raw material to continuously react with a carbon raw material to generate gas, which reaches a seed crystal substrate on which a silicon carbide single crystal grows.
In another aspect of the present invention, there is provided a method for producing a silicon carbide single crystal comprising allowing a silicon raw material to react with a carbon raw material in a reaction crucible to generate reaction gas, that reaches a seed crystal substrate on which a silicon carbide single crystal grows, characterized in that the silicon raw material is continuously fed onto the carbon raw material which is maintained at a temperature such that carbon is allowed to react with silicon in a molten state or a gaseous state to generate the reaction gas.
In the above-mentioned methods, it is preferable that the silicon raw material of a finely divided particle form is fed onto the carbon raw material of a finely divided particle form, and that the carbon raw material is maintained at a temperature of 1,900xc2x0 C. or higher. The generated gas is preferably passed through an additional carbon material to the seed crystal substrate, said additional carbon material being disposed midway along a path of the generated gas reaching the seed crystal substrate.
In a further aspect of the present invention, there is provided an apparatus for producing a silicon carbide single crystal comprising a reaction crucible, and a seed crystal substrate disposed in the reaction crucible, on which substrate a silicon carbide single crystal grows, said apparatus further comprising means for maintaining a carbon raw material placed in the reaction crucible at a temperature such that carbon is allowed to react with silicon in a molten state or a gaseous state to generate the reaction gas, and means for continuously feeding a silicon raw material onto the carbon raw material placed in the reaction crucible.
The apparatus of the present invention preferably further comprises means for feeding the silicon raw material of a finely divided form onto the carbon raw material of a finely divided form; or means for placing an additional carbon raw material thereon, which is disposed midway along a path of the generated gas reaching the seed crystal substrate; or means for feeding the carbon material from the outside of the reaction crucible.